Pyrometallurgical smelting processes involve production of metallic particles from their precursors such as oxides, sulfides or the like by reaction at high temperature with gaseous or solid reductants and separation of the resulting molten metallic and non-metallic phases. If the concentration of the metallic particles in the molten bath is low, additional metallic particles may be added to the bath or additional metal may be melted in the hearth of the furnace in order to enhance the separation and collection efficiency for the metallic particles. It is not necessary that the additional metal is the same as the metal to be recovered; the principal requirement is that the metals form a molten alloy at bath temperature. A proposed mechanism for enhancing collection involves increasing the probability of contact between particles causing agglomeration of metallic particles in the molten bath so that larger particles form. The larger particles settle out of the slag phase more quickly and form, or collect in, a molten metal phase layer. The probability of contact and agglomeration efficiency for metal particles can be enhanced by increasing the turbulence and velocity of circulatory currents in the molten bath of metallic and non-metallic phases. However, if the turbulence is too great and the circulation currents too vigorous, the agglomerated metal particles may become entrained in the slag phase preventing efficient separation. The metallic layer may be separated from the slag layer by methods well known to those skilled in the art, such as settling and tapping the metallic layer from the furnace.
These processes can be carried out in plasma electric arc furnaces for separation of finely divided particles of precious metals or precursors thereof, from mineral concentrates, non-metallic substrates or supports used for precious metal catalysts. Usually the concentration of precious metals in the feedstock to the furnace is relatively low, such as less than 1% or even less than 0.1%. It is well known to use non-precious metals such as iron, copper, nickel, lead, as collector metals to enhance the collection efficiency of the finely divided precious metals. The plasma electric arc furnace is especially suited for separation of metals with high melting points such as platinum group metals and ferro-alloys from feedstocks having high melting points because of the high heating intensity of the plasma arc flame. An example of a plasma electric arc furnace with a transferred arc flame that can be used in pyrometallurgical processes, is described in U.S. Pat. No. 4,685,963 to Saville.
A goal sought by inventors of plasma arc furnaces was to expand the volume of the high temperature arc so that the efficiency of heat transfer to solid, liquid or gaseous phase feedstocks could be increased. This was achieved in part by Tylko with a transferred plasma as described in British Patent Specification 1,390,351. Tylko moved the tip of the plasma torch in a circular path by precessing the torch through a spherical bearing located at the vertical axis of the furnace. The plasma flame formed a conical volume of high temperature ionized plasmic gas through which solid feedstock particles travelled and carried out reactions and melting resulting in production of metallic and slag layers in the bath at the base of the furnace.
Hubweber in U.S. Pat. No. 4,651,326, recognized that the high heating intensity of the plasma arc flame could cause local overheating and vaporization of the molten bath in a furnace at the impingement zone of the flame, and taught a mechanism for moving the tip of the torch universally to prevent this occurring.
Saville in U.S. Pat. No. 4,685,963 teaches that the locus of the tip of a plasma torch can be adjusted to describe a circle and produce a superheated puddle in the molten bath wherein vigorous thermal currents and physical agitation of the bath by impingement of the plasma gas can increase the agglomeration of metal particles suspended in the slag and the efficiency of recovery of the particles into the metal layer.
While adequate results for separation of metallic particles using prior art plasma arc furnaces have been achieved, further improvements in the efficiency of recovery of metallic particles remains a desired goal.